The present invention relates to methods of delignification of wood pulp and more particularly, to improvements in alkaline-oxygen delignification of wood pulp.
Processes for producing paper include a pulping stage in which a major part of delignification is realized. A subsequent bleaching stage is utilized in the production of white paper. In this latter stage, the delignification process is continued and results in the purification of cellulose fibers which is necessary in the production of white or dyed paper. In bleaching processes designed to enable production of such paper, lignin content in the pulp is reduced by molecular fragmentation and/or dissolution while degradation of cellulose is avoided as far as possible. However, although a "whiteness" bright to the eye is to be developed by bleaching, other characteristics such as tensile strength, burst, folding endurance, etc., and brightness stability must also be maintained.
Bleaching of wood pulp is comprised of a sequence of steps designed to (1) prepare pulp for solution of ligneous compounds not previously dissolved by chemically fragmentizing lignin polymer, (2) dissolve fragmentized lignin, and (3) develop brightness in the pulp by using oxidative decolorizing reagents. The first step is often attained by chlorination, the second by `extraction` with alkaline solution and the third by using hydrogen peroxide, chlorine dioxide, hypochlorite, etc. Although the number and nature of such stages will vary in different bleaching sequences, the above functions must be performed to achieve final paper strength and brightness characteristics.
Widespread usage of chlorine for lignin removal has resulted from the excellent specificity of chlorine for lignin, and the rapidity of reactions results in only minimal attack on carbohydrates, i.e. cellulose. Although the foregoing attributes of chlorination stages facilitate pump bleaching, substantial quantities inthe form of color, BOD (biological oxygen demand), COD (chemical oxygen demand) and chlorides are contained in the effluent of bleaching stages. While such effluents have previously been simply discharged to streams, lakes, etc., environmental statutes and regulations now severely restrict the permissible quantities and concentrations of such pollutant containing discharges. Furthermore, it has been found that effluent of the extraction stage following a chlorination stage also contains substantial quantities of color BOD, COD, chlorides and toxic chlorinated organic compounds, the discharge of which into natural bodies of water is obviously highly undesirable and generally prohibited by environmental codes and regulations.
In order to treat the pollutants noted above from a bleaching sequence, it has been proposed to utilize (a) lime precipitation to remove color, (b) aerated oxidation lagoons to lower BOD and (c) demineralization to eliminate chlorides. However, these treatment stages result in increased capital costs of a given bleaching sequence as well as higher operating costs.
As a partial or even complete alternative to chlorine and caustic extraction stages of a bleaching sequence, oxygen has been utilized to render lignin soluble in an alkaline solution to thereby effect pulp delignification. The advantages of oxygen delignification are seen to reside in reduced levels of pollutant discharge, and the avoidance or reduction in production of chlorinated organic compounds. Additionally, there is a reduction in energy required to refine an oxygen pulp for paper making. Oxygen delignification processes are generally classified as either a high or low consistency type. In the former, dry pulp fibers comprise at least 20% of the moist pulp or slurry, while in the latter, dry pulp fibers comprise less than approximately 5% of such slurry. It is also known to delignify pulp in a medium consistency slurry comprised of approximaterly 6-15% dry pulp fibers.
In a high consistency oxygen delignification process, pulp fibers are covered with a thin film or layer of an alkaline solution which is permeated by oxygen gas. The reaction rate is determined by the result of such permeation or gas diffusion. In essence, fibers bearing an alkaline film are virtually "fluffed" in a reaction zone containing oxygen gas. The high consistency oxygen delignification process is not without its disadvantages, including high capital costs. Also, the availability of oxygen can lead to scorching or fiber degradation which is minimized by limiting the extent of oxygen delignification to about a 50% Kappa Number, reduction.
In low consistency oxygen delignification processes, oxygen is present as a dissolved material in the alkaline solution. A generally more uniform, higher quality pulp is producible by low consistency processes. However, such processes have heretofore required extensive agitation due to difficulties of dissolving oxygen in alkaline solutions. The solubility of oxygen in alkaline solutions is relatively low and it has been found that dissolved oxygen levels increase with increased rates of fluid agitation and surface exposure to gaseous oxygen.
Accordingly, a clear need exists for oxygen pulp delignification processes, which avoids the necessity of chlorine for delignification but which enable production of final pulp products having desired physical characteristics without incurring increased overall process and capital costs.